Fiber reinforced plastic (FRP) is prepared by impregnating synthetic resin such as epoxy resin, vinylester resin, phenol resin, and so on into reinforcing fiber such as glass fiber, carbon fiber, aramid fiber and so on. The fiber reinforced plastic is generally used in various areas which require electric-insulation, thermal-insulation, and non-magnetic properties, and is also used as a structural reinforcing material in civil engineering and construction industry. The fiber reinforced plastic is also used to manufacture an article such as a bolt or a nut for fixing substances.
FIGS. 3 and 4 are drawings for illustrating a method for producing a FRP bolt in accordance with a conventional method. As shown in FIG. 3, in the conventional method, reinforcing long-fiber 12 is impregnated with liquid-phase synthetic resin, and is pultruded with a thermoforming die 20 while being transported in parallel. The pultruded and synthetic resin impregnated fiber is hardened to form a FRP round bar 14. The hardened round bar 14 is cut with a cutter 22 to a predetermined length. As shown in drawing A in FIG. 4, the surface of the FRP round bar 14 is ground with a bite 24 of a screw cutting device to form a screw thread of a FRP bolt 16. As shown in drawing B in FIG. 4 which is a longitudinal sectional view of the produced FRP bolt 16, the produced FRP bolt 16 has long-fiber 12 which is unidirectionally aligned along the axis of the bolt 16. Thus, the screw thread of the FRP bolt 16 is easily damaged by an external force, and the FRP bolt 16 is not suitable for connecting structures or substances in which a strong force is applied. In other words, as the surface of the FRP round bar 14 is ground away to form the screw thread of the FRP bolt 16, the reinforcing long-fiber 12 (filament) in the screw thread is cut off, the mechanical strength of the long-fiber 12 is deteriorated, and the tensile strength of the FRP bolt 16 is also deteriorated.
Meanwhile, in order to increase the strength of screw thread of a FRP bolt, Japanese Patent Laid-Open No. H07-279933 discloses a bolt having an internal body and a screw thread which are made of different materials from each other, wherein the screw thread covers the internal body and is produced by a molding process. However, the resin impregnation process and the hardening process in a mold are complicated in the method, and thus the productivity of the bolt is not desirable. Alternatively, Japanese Patent Laid-Open No. H06-114859 discloses a method for producing a bolt including the step of forming a round bar made of reinforcing fiber impregnated with thermoplastic resin with the first mold, and forming the round bar into a bolt having a screw thread with the second mold. However, this method requires a molded preform made of thermoplastic resin, and the preform should be molded again. Thus, the method is not desirable in its productivity. Furthermore, for producing a screw thread of a FRP bolt by a molding process, several molds should be prepared for each type or size of the bolt, which result in the high production cost and the low productivity.